Electrolytic cells are employed in both liquid crystal and electrochromic displays. In an electrochromic display, the passage of an electric current through the cell to display electrodes causes an electrochromic material present on or near the electrodes to undergo a distinctive colour change.
One well known type of electrochromic display, an early example of which is described in U.K. Pat. No. 1,376,799 (Philips), employs an electrochromic substance in solution which is transparent while dissolved but coloured when electrodeposited upon a display electrode. The coloured and transparent states form a redox pair so that the deposited coloured material can be electrolytically removed by reversing the current direction. Perhaps the best known class of electrochromic substances employed in such displays are the bipyridinium compounds known as "viologens". The di-cations of the viologens such as the 1,1'-di-heptyl-4,4'-bipyridinium di-cation are frequently transparent in solution and can be reduced electrochemically to the radical cation which is violet coloured. In the presence of a suitable anion, such as bromide, phosphate or phosphate/hypophosphite mixtures, the coloured viologen radical salt precipitates out on the cathode.
Somewhat less well known electrochemical displays, which are not strictly electrochromic, involve deposition of, for example, metal films on transparent electrodes to render these electrodes opaque. Early work in the field is described in an article by J Mantell and S Zaromb (J Electrochem Soc Vol 109, pp992-993, 1962). More recently, a display based on the reversible electroplating of black silver has been described in an article entitled "Electrolytic display" by Duchene et al. (Proceedings of 1978 Biennial Display Research Conference, Cherry Hill, N.J., pp34-37.)
Electrochromic and other electrolytic deposition displays, as discussed above, may be viewed or projected in either transmitted or reflected light. Differential absorption of light between the written and unwritten areas determines the achievable contrast ratio which is an important measure of display performance. The contrast ratio is a function of the absorption coefficient of the material and the thickness of material deposited. The article by Duchene also mentions an anomalous enhancement of absorption attributed to scattering of light by the silver deposit.
In the case of viologen based displays employing white silver electrodes, it has proved difficult to attain photopic contrast ratios of more than 3:1 with the deposition of thicknesses of viologen equivalent to 2 mCcm.sup.-2. To deposit a greater thickness encourages the formation of irreversible deposits and, because of the increased writing time, imposes a significant limitation on the performance of a matrix addressed display.
An article entitled "Projection Display with Electrochemically Created Grating Structures" by W B Pennebaker (IBM Technical Disclosure Bulletin Vol 23, No 1, June 1980, p397) describes an alternative diffraction based technique for achieving contrast in electrolytic displays employing either viologen or silver as the colouring material. Individual picture elements ("pels") of an addressable array consist of electrodes which are formed with a grating structure such that deposition of viologen or silver reveals or enhances the grating. Thus, written pels, when illuminated, diffract incident light to a projection system whereas unwritten pels do not. Contrast thus depends purely on the relative intensity of the diffracted order to the background. The contrast improvements offered by such a system are only realised at the expense of a relatively complex grating structure on each pel surface.
To complete the review of the prior art, the contrast producing mechanisms in liquid crystal displays will be considered. Field effect liquid crystal materials rely on the polarization of light by the liquid crystal material in written areas of the display. Field effect displays must be viewed in transmission and require two polarisers. Another type of liquid crystal material relies on dynamic scattering of light by the liquid crystal material and can operate in reflected light. The basic principle of producing contrast in dynamic scattering displays is to illuminate the display from an angle which makes the scattered (diffusely reflected) image visible to the observer while deflecting the specular image out of the field of view. Examples of this technique may be found in U.S. Pat. Nos. 3,947,091 (RCA) and 3,992,081 (Suwa Seikosha).